im a newfag when it comes to electronics, and im a bit scared of li-ion cells, so i thoughtvid come and see if i was doing this right.
ive had an old laptop battery lying around and one of those cheap usb battery banks, so i decided to expand it a little. i plan on wiring up all of the batteries in paralell and using them as one cell with the charging circuit. am i going to blow anything up?
forgot to mention. i'm charging all of the cells individually before wiring them up together, so that theyre all balanced.
>>1013664
Your batteries are going to die fast. It's generally bad practice to wire batteries in parallel. You never see any battery-powered electronics with this kind of arrangement.
Cell load balancing is the biggest challenge. I wouldn't use old cells that are likely no longer chemically identical.
Look up info on electric bike forums. Those dudes are pro at building balanced packs, without as much risk as venting with flame.
>>1014053
You mean everyone is doing bad practice than, nice. 3s 3p is standard for laptop batteries
![dscn2490[1].jpg](https://i.imgur.com/IOcyiw6.jpg "dscn2490[1].jpg")
>>1014053
It's absolutely fine to wire cells in parallel.
Charging: least-charged cell takes up the slack and charges; charged cells present high resistance and won't overcharge so long as an uncharged cell is present. Current-based cutoff works as normal.
Discharging: most-charged cell takes up the slack and discharges; least-charged cells won't overdischarge (and will actually charge a bit) because terminal voltage is higher than their own voltage.
It's series you have to worry about. If you're discharging a long string of lithium cells in series, it's possible that one cell can be in undercharge, whilst the rest of the cells are charged sufficiently that the string still shows a nominal voltage. So one cell is being ruined, but the battery looks fine.
Conversely, if you just apply a charging voltage to the whole string, the reverse can happen: one cell with a diminished capacity gets overcharged and goes on fire, whilst the string as a whole looks fine.
Proper batteries don't let either of these things happen, because their protection circuits monitor the voltage across each cell (or group of parallel cells) individually.
>>1014123
It's not a standard, but a lot of laptops do do it. Wiring the whole lot in series is also common; Dell, for example, does this with their bay batteries. This design does require a more complex charging circuit, as it needs to charge eight groups of cells instead of just three, and it needs to monitor every cell for undercharge.
But laptop batteries discharge through a protection circuit, not a wire. OP's real problem is that his cells are not protected, and if he over-discharges them, they'll stop working. They won't go bang or anything, they'll just drastically lose capacity as the anode dissolves in the electrolyte.
>>1013664
>am i going to blow anything up?
Maybe.
If "the charging circuit" provides overcharge and undercharge protection then no.
Otherwise yes.
>>1013664
>charge, then connect
no. Exactly the other way, discharge, then connect, preferably select cells that are closest together on open voltage.
This way if you fuck up, nothing will go boom, because there will be no energy left.
Recharge right after finishing work.
>>1014135
>Proper batteries don't let either of these things happen, because their protection circuits monitor the voltage across each cell (or group of parallel cells) individually.
You're ignoring the fact that OP's setup doesn't have a protection circuit and all five of his cells are wired in parallel instead of in pairs like in your example. You're also ignoring the fact that all battery packs have some form of series arrangement of the cells.
In cheap battery chargers without any monitoring, there is a diode connected in series to each cell. In medium-priced battery chargers, the batteries are charged in pairs in series. In good battery chargers, each cell is monitored individually. In your example, it's obviously a cost-cutting measure for the manufacturer to wire the cells in pairs in parallel.
>>1016968
>You're ignoring the fact that OP's setup doesn't have a protection circuit
The thing with the LED is a protection circuit. >and all five of his cells are wired in parallel
Perfectly fine. You protect each parallel group.
>instead of in pairs like in your example.
So?
>You're also ignoring the fact that all battery packs have some form of series arrangement of the cells.
No, only the ones that need more than 3.7V nominal.
>In cheap battery chargers without any monitoring, there is a diode connected in series to each cell. In medium-priced battery chargers, the batteries are charged in pairs in series. In good battery chargers, each cell is monitored individually. In your example, it's obviously a cost-cutting measure for the manufacturer to wire the cells in pairs in parallel.
This... makes ~ no sense.
You can't individually balance cells in a parallel group. It's nonsensical.
>>1013664
>>1014135
If you are going to make a pack like this I recommend taping it once you are finished, wiggling could disrupt the connection otherwise.
Things that will make a li-ion explode
•getting punctured
•exposure to high heat
•charging above 4.2 volts
When selecting batteries for your pack know that it is only as strong as your weakest link
>>1013664
> am i going to blow anything up?
They're pretty dangerous without any protection circuits built in. I was once working with just two 18650s in series and accidentally let the wires lightly touch. The wires welded themselves together and within a few seconds the cells were glowing and burning a hole through my workbench.
>>1018662
What's with the resistors?
Do you need a protection circuit for driving a large-ish dc motor from lion? I want to convert one from using a 12V lead-acid.
>>1019118
>resistors rated at 4.2V
Please tell me you're joking. At least zeners would make sense.
>>1019270
1k ohm resistor my bad
>>1019118
the resistors are used as fuses; burning out when their calculated current is exceeded
you actually pick the wattage slightly under maximum
>>1019118
>>1019270
>>1020078
That's called a thermistor. And it's for overcurrent. PTC thermistor in this case. (http://www.littelfuse.com/products/resettable-ptcs.aspx)
18650 often come with a protection circuit onboard. That's what detects exceeding voltage and cuts off the battery.
>>1013664
>as one cell with the charging circuit
How are you going to tell if the charging terminated? The simplest chargers just sense if the current is less than 5/10mA; for multiple batteries, you might never reach that, and the charger will always indicate 'charging'.
>as one cell with the charging circuit
That'll probably take very long to charge, unless you make a customized charging circuit, but if you were able to, you'd probably not need to ask us for the advice.
